1. Field of the Invention
The present invention relates to a dashboard of a motor vehicle having a microphone which is connected to a further electronic unit.
2. Description of the Related Art
The use of fixed microphones for providing input to electronic units is know in the art. The electronic unit may be, for example, a mobile phone or a voice recognition system which is connected to a system for the voice control of motor vehicle functions or other functions. Particularly in the case of a voice recognition system, it is important for the voice instructions which are uttered by a person to be understood satisfactorily by the system. For this reason, a sufficiently good signal-to-noise ratio is necessary. However, in motor vehicles there is a large amount of background noise. This includes, for example, not only the noises of the engine and of the movement but also music from an audio system or the voices of other vehicle occupants. The positioning and design of the microphone is therefore highly significant.
The signal-to-noise ratio for the voice signals of a vehicle occupant can be improved in various ways. Because the level of the voice signal decreases as the distance from the speaker's mouth increases, specifically by approximately 6 dB when the distance is doubled, it is advantageous to arrange the microphone as close as possible to the speaker's mouth. Various arrangements of a microphone in the motor vehicle are known.
It is known to integrate a microphone into the inner roof lining. This has the advantage that the microphone is arranged near to the speaker's mouth at a distance of approximately 30 to 40 cm, and therefore a good signal-to-noise ratio is obtained. However, the routing of cables and the restriction in terms of design are disadvantages of this solution.
Furthermore, it is known to provide a microphone on the seat belt. Such belt-mounted microphones are, with their distance of approximately 10 to 20 cm, indeed very close to the driver's mouth, but when the belt is put on they do not automatically assume the correct position and are therefore not very convenient to use.
DE 198 00 442 A1 discloses an embodiment in which a microphone is integrated into the sun visor of a motor vehicle. A disadvantage of this embodiment is that before the microphone is used, the sun visor must always first be moved into the correct position. EP 0 635 395 B1 discloses an embodiment in which a microphone is integrated into the interior mirror of a motor vehicle. A disadvantage with this embodiment is not only the routing of the leads but also in particular the fact that the microphone is further away from the driver than in the previously mentioned solutions.
Furthermore, DE 35 130 017 A1 discloses an arrangement in which a built-in microphone is arranged in the center console or the dashboard of a motor vehicle. Such an arrangement has the disadvantage that it is comparatively far from the speaker and therefore also picks up background noises, for example, the voices of the other vehicle occupants, to a high degree.
M. Dahl et al.: "Simultaneous Echo Cancellation and Car Noise Suppression Employing a Microphone Array", ICASSP97, Munich, Vol. 1, pp. 239-242, discloses the use of a microphone line in the motor vehicle. A disadvantage here is that the microphone line has to be made up of a multiplicity of individual microphones and downstream processing devices in order to achieve the desired accentuation of the voice signals. This embodiment is therefore structurally complex and expensive.
Such a microphone line is classified as a directional microphone. Generally, directional microphones permit signals to be picked up selectively from sound sources in a preferred direction (useful signals), sound sources which do not lie in the preferred direction of the microphone (interference signals) are reduced in terms of their signal level, and thus are not received as well. Various embodiments of directional microphones are known for other applications. The useful signals can be accentuated here in three ways:
Simple directional microphones: known embodiments of microphones include sound pressure receivers and high-speed sound receivers or combinations thereof. Sound pressure receivers receive sound from all directions uniformly. High-speed sound receivers have, on the other hand, a directional characteristic in the form of an eight. Combinations of the aforesaid receivers can also have a directional characteristic in the form of a so-called kidney or a super kidney. An appropriate directional characteristic can increase the sensitivity of the microphones in free space in the preferred direction by up to 8 dB.
When used in a motor vehicle, the microphones must always be placed as flush as possible with a part of the vehicle. They can therefore only be used as boundary surface microphones with a directional characteristic parallel to the surface of the part of the vehicle. The consequence of this is that the speaker is no longer located precisely in the preferred direction and only relatively small accentuations of the level of up to approximately 5 dB can be achieved.
Microphone lines: in a way analogous to the method of elementary radiator synthesis known from antenna technology, it is possible for a microphone line in which a plurality of individual microphones are arranged spatially offset to sample the sound field at various points. The sampled signals are then summed with a suitable propagation time delay in such a way that the sound signals which are incident from the preferred direction of the line are summed in the same phase and thus amplify one another, while the signals from other directions with different phase positions are summed and thus cancel one another out entirely or partially. The use of microphone lines in a motor vehicle is known from the above-mentioned publication.
Microphones with a lobe-shaped directional characteristic: this group includes the slotted-tube type microphone or tubular directional microphone which is composed in its general form of a tubular element which is closed at both ends. The closure, at the "far" end a microphone, contains a capsule which picks up the sound in the tube. Sound can enter the tube through a lateral slot or through a respective row of small openings. If the sound in the tube is considered as a function of the position of an external sound source, it is found that only sound from sources in a position to which the "near" end of the tube points arrives at the microphone capsule in the same phase after entering through the slot and passing on through the tube, while in all the other directions of incidence propagation time differences, and thus reductions in the level of the summed signal, occur. This means that a slotted-tube type microphone operates like a line of microphones of a comparable length, but to form it just one microphone capsule with processing means connected downstream is necessary, and it can therefore be implemented with comparatively little effort.
A directional microphone with a lobe-shaped directional characteristic can also be embodied as a parabolic mirror microphone, like a parabolic antenna, that is to say the accentuation of signals from a preferred direction is achieved in that the signals from the preferred direction of the parabola are reflected onto the focal point of a parabola and arrive there with the same phase position and propagation time. At the focal point of the parabola, a microphone capsule which picks up the sound is mounted. Sound which does not come from the preferred direction in preferably not reflected into the focal point and is therefore picked up with a lower level than sound from the preferred direction.
A disadvantage with commercially available parabolic mirror microphones and slotted-tube type microphones is that owing to their dimensions they are difficult to position in a motor vehicle. Slotted-tube type microphones are provided by various companies with tube lengths of 0.1 to 0.4 m. Parabolic microphones require a sufficiently reflective and focusing surface. Excessively short tubes and too little surface make the directional characteristic too small.